Question

word color;

crap = ((color & 0xffc0) >> 1) | (color & 0x1f)

I have this shifting code. I have no idea what the purpose is of this code, but I suspect it has something to do with switching between 555 to 565 colors.

How would I create a function that does the exact opposite as the code above? That converts the variable back to the original color number?

Answer 1

I suspect it has something to do with switching between 555 to 565 colors.

Yes, that’s what it’s doing. color will be the 565 representation, and crap the corresponding 555 representation.

Lets look at what each operation does. I’ll refer to the three fields as a (5 bits), b (6 bits, transformed to 5) and c (5 bits).

(color & 0xffc0) clears the bottom 6 bits of the word, removing c and the least significant bit of b, preserving a and the most significant 5 bits of b.

>> 1 shifts these bits right, so we now have a (5 bits), b (5 bits), and an empty 5-bit field.

(color & 0x1f) clears all the bits except for the bottom 5 bits – that is, it preserves ‘c’ and removes the other fields.

Finally, | combines the two values, giving a and b from the left-hand side, and c from the right-hand side, each in 5 bits.

How would I create a function that does the exact opposite as the code above? That converts the variable back to the original color number?

color = ((crap & 0xffe0) << 1) | (crap & 0x1f);

Note that the least significant bit of b is now zero, whatever it was to start with. That information was lost in the first transformation, and can’t be recovered.